No 6) An introduction to gravity: (See OP if you wish to discuss further) https://www.facebook.com/groups/2205176794/permalink/10155798994861795/ No 6a) Gravity is towards the center of mass ------------- If we were in space and started from scratch. Say we had some water to the right of us and some rock to the left of us. They would attract each other. So the left would be down and the right would be up? Why would below us be down. It's two totally separate objects with nothing below them. Why is below them down? ------------------------ It ISN'T. There is no up or down in space! Down is towards the center of mass! (See diagram No 6a) No 6b) How gravity pulls you: - Where ever you are, in the Earth, the net mass pulling you is the sphere below you. The shell above you cancels out. (See picture No 6b-1) (See video No 6b-2) https://www.youtube.com/watch?v=s94Gojs3Ags No 6c) Why if you measure yourself at different places around the world with a spring scale or a balance scale, do you get two different answers? Why does the balance scale always give the same answer and the spring scale varies? GRAVITY! One is comparing masses and the other is measuring the force of gravity! (See diagram No 6c) ----------- The science behind it; - When you are using a balance scale, you are comparing one mass to another. Literally comparing two items with the same amount of "stuff"/mass. - When you are using a spring scale, you are measuring the pull of the Earth on your object. Since this pull varies around the planet, in some places you will be lighter and in others you will be heavier. No 6d) F=9.81m/s^2*Mass, is a special case. The general formula for the force of gravity is; F=G(m1*m2)/r^2 Most things are constant except for the objects mass, m2. When we put in the values we get; F=9.81*m2 (Mass, in Kg's of the object) That is F=MA. *** The entire mass of the Earth pulls on every Kg with 9.81 newtons of force, at the Earth's surface. *** ----------- Roughly speaking, gravity is a constant on the surface if the Earth. Why? ... the Earth is roughly a sphere, so everywhere on the surface is identical. - Of course when we measure in fine detail, we find it isn't a perfect sphere, but it is close enough for every day use. Note: Kg's is NOT weight, it is a MASS, a measure of how much stuff is in an object. No 6e-1) Density is a how much mass there is per unit volume. So it is Kg's per volume. Never lb/volume. No 6e-2) lb is weight, Kg's is not: If you want weight in metric, then it is in NEWTONS. e.g. 9.81*m2 = 9.81 newtons/Kg No 6f) We should probably say towards each other, when talking about gravity, instead of up and down. Up and down are too subjective and can be misinterpreted, e.g. flat Earth.